Multi-faceted Roles of an Atypical Kinase RIOK2 in Erythropoiesis and Myelodysplastic Syndromes Project Summary Myelodysplastic syndromes (MDS) are the most commonly diagnosed neoplasms in the United States with a dismal survival rate. Furthermore, 1 in every 4 MDS patients is at risk of developing Acute Myeloid Leukemia (AML), a devastating hematologic malignancy of myeloid cells. The median age of MDS diagnosis is 60 years, thus making the only cure, stem cell transplantation unlikely in an older demographic with significant associated co-morbidities. Current therapies using lenalidomide and hypomethylating agents are effective in a subset of MDS patients but are also associated with side effects such as myelosuppression. Though our understanding of the etiology of MDS has increased substantially in the past decade, the role of the immune system in MDS pathogenesis is still greatly under explored. Thus, treatments targeting immune mediators have not been therapeutically tested in MDS. Based on our preliminary studies of a mouse model of haploinsufficient Riok2 expression (Riok2+/-) and MDS patient samples, we have identified a link between Riok2 haploinsufficiency-mediated myelodysplasia and induction of the immune cell-derived cytokine IL-22. Our data show that Riok2 haploinsufficiency has a direct negative effect on erythroid progenitor differentiation and an indirect effect by inducing erythropoiesis- suppressive IL-22 from T cells. Additionally, Riok2 haploinsufficiency-mediated cell cycle changes lead to increased proliferation of myeloid cells. Importantly, RIOK2 mRNA expression is decreased in bone marrow cells from MDS patients as compared to healthy controls. We have also identified one dominant negative and 5 loss of function RIOK2 mutations that negatively impact erythropoiesis. Haploinsufficient deletion of Il22 in Riok2+/- mice reversed the erythroid differentiation defect. Our study is the first to identify the critical function of Riok2 in regulating erythropoiesis as well as the immune system, both synergistically leading to myelodysplasia. In this proposal, we aim to mechanistically unravel the critical role of Riok2 and IL-22 in MDS and test the efficacy of IL-22 inhibition on alleviation of the anemia and myelodysplasia seen in MDS. We will assess in detail how loss or mutation of RIOK2 affects ribosome biogenesis, erythropoiesis, myelopoiesis, and IL-22 production. Furthermore, we will also test whether IL-22 inhibition in Riok2+/- mice is a therapeutic strategy to alleviate the anemia and myelodysplasia seen in MDS. Moreover, the high-throughput transcriptome and proteome analyses of this novel mouse model of Riok2 haploinsufficiency-mediated MDS proposed here are expected to reveal additional therapeutic targets, immune as well as non-immune, that can potentially be targeted for MDS treatment. This knowledge should pave the way for devising a new generation of therapies that will improve the outcome of disease for MDS, AML and anemia patients. Targeting IL-22 may encourage more effective immune- based treatments that, either as single agent therapy or in combination with current therapies, will improve the prognosis and quality of life of MDS patients.